A transmission section of a communication apparatus, such as a portable telephone and a wireless LAN, is required to operate at low power consumption while maintaining the accuracy of transmission signals, regardless of the magnitude of the output power. In particular, the amount of power consumed in a power amplifier of the final stage of the transmission section of the communication apparatus is a half or more of the power consumed in the entire communication apparatus, and hence the power amplifier needs to have high power efficiency.
In recent years, a switching amplifier has been attracting attention as a power amplifier which is expected to have high power efficiency. On the assumption that a pulse waveform signal is to be inputted, as an input signal, into the switching amplifier, the switching amplifier power-amplifies the pulse waveform signal while maintaining the waveform of the signal. The pulse waveform signal that is amplified by the switching amplifier is radiated into the air from an antenna after the frequency components of the pulse waveform signal other than desired frequency components are sufficiently reduced by a filter element.
FIG. 1 shows a class-D amplifier which is a typical example of the switching amplifier. The class-D amplifier includes a configuration in which two switch elements are inserted in series between a power supply and the ground. Complementary pulse signals are inputted into the two switch elements as opening/closing control signals so that only one of the switches is turned on. The output of the amplifier is controlled in such a manner that, when the switch on the power supply side is turned on and when the switch on the ground side is turned off, a voltage equal to the power supply voltage is outputted, and that, when the switch on the power supply side is turned off and when the switch on the ground side is turned on, a voltage equal to the ground potential is outputted.
This class-D amplifier does not need any bias current, and hence ideally no power loss occurs. Note that the switch element can be configured by an MOS (Metal Oxide Semiconductor) field-effect transistor or a bipolar transistor.
As a configuration example of an entire transmitter using the class-D amplifier, FIG. 2 shows a configuration described in Non-patent Literature 1. For example, when W-CDMA (Wideband-Code Division Multiple Access) is taken as an example, a radio signal is generated as a multi-bit signal of 10 bits or more by, for example, a digital baseband (hereinafter referred to as a DBB) processor. On the other hand, the input signal of the class-D amplifier is a pulse waveform signal. Since the pulse waveform signal can transmit only a one-bit signal, the output signal from the DBB processor needs to be converted into one-bit signals beforehand. In this configuration example, in order to maintain desired noise characteristics near a frequency band of a desired radio wave, a ΔΣ (delta sigma) modulator is used as means to convert the multi-bit signal into one-bit signals. With this configuration, it is possible that, while desired noise characteristics of the radio signal are maintained, the radio signal is converted into a pulse waveform signal, so as to be inputted into the class-D amplifier.
Patent Literature 1 describes an invention in which the level of a signal is adjusted before the signal is subjected to delta-sigma modulation.
Patent Literature 2 describes an invention which relates to a circuit for performing analog-to-digital conversion by using a plurality of delta-sigma modulators arranged in parallel with each other.